1. Field of the Invention
The present invention relates to a heating apparatus provided with a heat-generating unit using a shearing force, for increasing a temperature of cooling water for cooling a water-cooled engine.
2. Description of Related Art
Conventionally, as a heating apparatus for a vehicle, there has been generally known a hot water type heating apparatus for heating a passenger compartment, in which cooling water for cooling a water-cooled engine is supplied to a heater core disposed in a duct, and air heated while passing through the heater core is blown into the passenger compartment by a blower to heat the passenger compartment.
Recently, an improvement of an engine efficiency is highly demanded for an engine mounted on a vehicle; however, when the engine efficiency improves, heat loss decreases; and therefore, the cooling water for cooling the engine cannot be heated sufficiently. Further, in a case of a vehicle having a diesel engine or a lean burn engine, the heat amount generated by the engine is too small to heat the cooling water sufficiently. In the vehicle where the heat amount generated by the engine is small, a temperature of the cooling water in the cooling water circuit cannot be maintained at a predetermined temperature (e.g., 80.degree. C.), there occurs a problem in that a heating capacity for the passenger compartment is insufficient.
To overcome such a problem, as disclosed in JP-A-2-246823 or JP-A-3-57877, there has been conventionally proposed a heating apparatus for a vehicle, in which a heat-generating unit using a shearing force, for heating cooling water supplied from the engine to the heater core is disposed in a cooling water circuit, and when a temperature of the cooling water in the cooling water circuit, detected by a cooling water temperature sensor, is lower than a set cooling water temperature, the heat-generating unit is operated to improve the heating capacity.
The heat-generating unit transmits a rotational driving force of the engine to a shaft through a belt transmitting mechanism and an electromagnetic clutch, a heat-generating chamber is formed in a housing, and a cooling water passage is formed at an outer periphery of the heat-generating chamber. Further, a rotor which rotates integrally with the shaft is disposed in the heat-generating chamber, and a shearing force generated by a rotation of the rotor is applied to viscous fluid such as silicon oil sealed in the heat-generating chamber to generate heat. The cooling water is heated by the generated heat. That is, the heating apparatus for a vehicle uses a rotational driving force (driving torque) applied to the rotor of the heat-generating unit as an auxiliary heat source for heating operation.
In the heating apparatus for a vehicle, provided with a heat-generating unit as described above, it may be conceived that, a refrigerant evaporator of a refrigeration cycle is accommodated in a duct at an upwind side of a heater core, air flowing in the duct is temporarily cooled by the refrigerant evaporator while a refrigerant compressor is operated, and then is re-heated by the heater core to prevent a front windshield of the vehicle from being frosted or clouded.
In winter season, to prevent the front windshield of the vehicle from being frosted or clouded, an electromagnetic clutch is turned on to operate the refrigerant compressor, and further, to improve the heating capacity, another electromagnetic clutch is turned on to operate the heat-generating unit. However, if both of the refrigerant compressor and the heat-generating unit are operated, a large torque is applied to the engine or the belt transmitting mechanism. Therefore, an engine stall may be caused due to a sudden increase of a torque applied to the engine, a fuel consumption ratio of the engine may be increased, and a running cost (fuel economical performance) may deteriorate.
Further, there occur problems that each portion of the heat-generating unit may receive a large shock, an abnormal noise (chattering noise) may be generated by a slip of a belt of the belt mechanism, a durability of each portion may deteriorate, or the like. To solve those problems, when the electromagnetic clutch is turned off to stop the operation of the refrigerant compressor, there occurs a problem that a defrosting performance of the front windshield lowers to deteriorate a safety of the vehicle travelling.